I know that fire in a few words is the exothermic reaction of a fuel with an oxidizing agent, but I can't fully understand what exactly happens to piece of wood when it is ignited. How do molecules start producing a flame? In other words, what is the chemistry behind the production of flame?
https://www.youtube.com/watch?v=B0E4PX3e3RE It feels somewhat weird to answer my own question but I think this video describes exactly what I wanted. As it supports, when heat is applied to a piece of wood, some bonds of the molecules that make up wood, break and thus different compounds are formed. These compounds are not held back by some force and so they are released in the air. When these compounds meet atmospheric oxygen, under heat (=energy), they burn and thus more heat is released along with carbon dioxide and water. That stage can be described as ignition. Finally, this produced heat is able to preserve the fire.